Re: Showing primitive index scan count in EXPLAIN ANALYZE (for skip scan and SAOP scans)

On Wed, Nov 27, 2024 at 8:36 AM Matthias van de Meent
<boekewurm+postgres@gmail.com> wrote:
I think we should divide them because by dividing the total buffer usage by the number of loops, user finds the average buffer consumption per loop. This gives them a clearer picture of the resource intensity per basic unit of work.
I disagree; I think the whole "dividing by number of loops and
rounding up to integer" was the wrong choice for tuple count, as that
makes it difficult if not impossible to determine the actual produced
count when it's less than the number of loops. Data is lost in the
rounding/processing, and I don't want to have lost that data.
I think that you're definitely right about this. I changed my mind (or
changed it back to my original position) recently, when I noticed how
bad the problem was with parallel index scans: nloops generally comes
from the number of workers (including the leader) for parallel scans,
and so it wasn't that hard to see "Index Searches: 0" with the latest
version (the version that started to divide by nloops). Obviously,
that behavior is completely ridiculous. Let's not do that.

The precedent to follow here is "Heap Fetches: N" (in the context of
index-only scans), which also doesn't divide by nloops. Likely because
the same sorts of issues arise with heap fetches.

Yes, you are right, I agree with both of you.

-- 
Regards,
Alena Rybakina
Postgres Professional